This proposal will address itself to the effects of pregnancy on the immune response in humans. The transplacental passage of antigen will be studied. Mutual sensitization to the same antigen between mother and offspring will be examined in order to detect active immunization of the offspring. Mothers will be immunized with tetanus toxoid during pregnancy to induce active, protective immunization in her offspring. In the course of this study, evidence will be obtained for the presence of serum factors in the mother and the neonate which can depress, or possibly stimulate, the immune response. If such factors can be clearly defined, we will proceed to isolate and characterize them. These studies will help to define the immunological status in pregnancy and lead to the development of methods for "immunological engineering" whereby the immunization of the mother can endow her child with protection against a variety of infectious diseases and possibly also against oncogenic viruses.